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  • 1
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    Universe, Vol. 5, Pages 168: The General Linear Cartan Khronon (2019)
    MDPI
    In: Universe
    Details
    Publication Date: 2019
    Description: A Cartan geometry of the General Linear symmetry is formulated by dividing out the displacements from the group. The resulting action is quadratic in curvature, polynomial in all the (minimal) variables, and describes an observer space that—in the symmetry-broken phase—reproduces the predictions of General Relativity in the presence of dark matter.
    Electronic ISSN: 2218-1997
    Topics: Physics
    Published by MDPI
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  • 2
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    Universe, Vol. 5, Pages 167: Disformal Transformations in Scalar–Torsion Gravity (2019)
    MDPI
    In: Universe
    Details
    Publication Date: 2019
    Description: We study disformal transformations in the context of scalar extensions to teleparallel gravity, in which the gravitational interaction is mediated by the torsion of a flat, metric compatible connection. We find a generic class of scalar–torsion actions which is invariant under disformal transformations, and which possesses different invariant subclasses. For the most simple of these subclasses we explicitly derive all terms that may appear in the action. We propose to study actions from this class as possible teleparallel analogues of healthy beyond Horndeski theories.
    Electronic ISSN: 2218-1997
    Topics: Physics
    Published by MDPI
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  • 3
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    Energies, Vol. 12, Pages 266: Optimal Scheduling of Combined Heat and Power Generation Units Using the Thermal Inertia of the Connected District Heating Grid as Energy Storage (2019)
    MDPI
    In: Energies
    Details
    Publication Date: 2019
    Description: A better integration across sectors is an essential element of 4th generation district heating and smart energy systems allowing to react to volatile renewable energy generation. This sector coupling enables to use more cost-efficient storage as storage prices differ for different forms of energy. Thermal energy for example can be stored in comparably cheap storage tanks. Besides such dedicated storage, the thermal inertia of a heating grid can be used as thermal storage as well. In this paper, a classic unit commitment optimization for scheduling of combined heat and power units not considering grid dynamics is extended to cover thermal dynamics of heating grids. First an outer approximation of the grid storage capabilities is developed. Second, a very efficient formulation for the storage dynamics of a heating grid is introduced and its capabilities are shown in a motivating case study. In this study additional savings of several thousand Euros per day are achieved using the thermal inertia of a heating grid as storage.
    Electronic ISSN: 1996-1073
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Published by MDPI
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  • 4
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    Universe, Vol. 5, Pages 142: Flat Connection for Rotating Vacuum Spacetimes in Extended Teleparallel Gravity Theories (2019)
    MDPI
    In: Universe
    Details
    Publication Date: 2019
    Description: Teleparallel geometry utilizes Weitzenböck connection which has nontrivial torsion but no curvature and does not directly follow from the metric like Levi–Civita connection. In extended teleparallel theories, for instance in f ( T ) or scalar-torsion gravity, the connection must obey its antisymmetric field equations. Thus far, only a few analytic solutions were known. In this note, we solve the f ( T , ϕ ) gravity antisymmetric vacuum field equations for a generic rotating tetrad ansatz in Weyl canonical coordinates, and find the corresponding spin connection coefficients. By a coordinate transformation, we present the solution also in Boyer–Lindquist coordinates, often used to study rotating solutions in general relativity. The result hints for the existence of another branch of rotating solutions besides the Kerr family in extended teleparallel gravities.
    Electronic ISSN: 2218-1997
    Topics: Physics
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  • 5
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    Universe, Vol. 5, Pages 143: On the Gauge Fixing in the Hamiltonian Analysis of General Teleparallel Theories (2019)
    MDPI
    In: Universe
    Details
    Publication Date: 2019
    Description: The covariant formulation of teleparallel gravity theories must include the spin connection, which has 6 degrees of freedom. One can, however, always choose a gauge such that the spin connection is put to zero. In principle this gauge may affect counting of degrees of freedom in the Hamiltonian analysis. We show for general teleparallel theories of gravity, that fixing the gauge such that the spin connection vanishes in fact does not affect the counting of degrees of freedom. This manifests in the fact that the momenta of the Lorentz transformations which generate the spin connection are fully determined by the momenta of the tetrads.
    Electronic ISSN: 2218-1997
    Topics: Physics
    Published by MDPI
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